1. Technical Field of the Invention
This invention relates generally to polyimide random copolymers. In particular it relates to polyimide copolymers containing ether linkages. Polyimide copolymers were synthesized using an equimolar amount of dianhydride and a combination of diamines. Additionally, copolymers of controlled molecular weight were synthesized.
2. Description of Related Art
Polyimides comprise a large family of heterocyclic polymers which were first prepared in the late 1950's. Since then a large volume of information has been generated concerning the synthesis, characterization, use, and physical and mechanical properties of these polymers. Polyimides have been extensively evaluated for aerospace and microelectronic applications because of their excellent thermal stability, low density, chemical resistance, and their good electrical and mechanical properties.
Polyimides are the first choice as material candidates for dielectrics because of their successful use in printed circuit boards, and planarization properties. Additional properties necessary for dielectric applications include: ease of processing, good adhesion, chemical resistance, low moisture uptake, thermal stability, low CTE, and cost effectiveness Rothman, L. B. J. Electrochem. Soc. 1980, 127, 2216!.
Polyimides must possess high heat resistance because device processing may require sealing, packaging, die bonding, wirebonding, and soldering. Thermal stresses occur if there is a mismatch in the CTE of the polyimide and the substrate, resulting in peeling and cracking. A low CTE minimizes these effects. Additionally, a low dielectric constant is required to minimize propagation delay, interconnect capacitance, and crosstalk between lines; this allows circuits to be run at a lower power input Monk, D. J.; Soane, D. S. In Polymers for Electronic and Photonic Applications; Wong, C. P., Ed.; Academic Press, Inc., 1993; p 119-165!. The dielectric constant of commercial polyimide film changes with absorbed moisture, reducing its overall performance, so a material with low moisture uptake is desirable.
Polyimides containing 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) were developed because of their low CTE. The first reports of a polyimide utilizing SAPPD appeared in 1993 St. Clair, T. L. U.S. Pat. No. 5,212,283, 1993 and Croall, C. I.; St. Clair, T. L.; Yumino, Y.; Mutoh, H.; and Ito, Y., "Polyimides Containing Squaric Acid Derivatives" presented at the Symposium on Recent Advances in Polyimides and Other High Performance Polymers, Reno/Sparks, Nev., Jan. 18-21, 1993!. The main focus of the patent was linear polyimides containing the cyclobutene-3,4-dione moiety. These polyimides exhibited glass transition temperatures greater than 500.degree. C. and adhered tenaciously to glass. The main focus of the presentation was polyimides containing the cyclobutene-3,4-dione moiety for space applications. Several polyimides were synthesized and subsequently exposed to 1 MeV electrons. Mechanical properties were evaluated before and after electron radiation exposure. Polyimides prepared with 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) possess relatively low coefficients of thermal expansion but have somewhat lower thermal stability than commercial polyimide film.
Although these polyimides prepared with SAPPD have an extremely low CTE, their high glass transition temperatures are undesirable. Also, these polymers have further undesirable characteristics such as low thermal stability, outgasing problems and lack of flexibility. Traditionally, polyimides with low CTE's also have high glass transition temperatures. For the microelectronics industry, low CTE polymers are required which also possess good dimensional stability with low glass transition temperatures to allow for processing. Typically, low CTE polymers are either crystalline or rigid, and thus not processable. By the present invention, copolyimides were developed that have both low CTEs, as well as, low glass transition temperatures. These copolyimides contain ether linkages and may include the 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione diamine.